Free-space optical (FSO) communications is a telecommunication technology which uses light beams propagating in free-space to wirelessly transmit data between two points. “Free space” and FSO means in air, outer space, in a vacuum, or a similar medium. FSO communications is useful when physical connections are not practical due to high costs or other considerations. In addition to terrestrial point-to-point networks, FSO communications can be used for building space-based links between satellites, unmanned aerial vehicles (UAVs), high-altitude platforms (HAPS), or from satellites/UAVs/HAPS to the ground. The wavelength used can be within the visible spectrum or within the infrared spectrum.
In a typical one-way laser FSO communication system between two aerial platforms or spacecrafts or a combination of the two, laser terminals are deployed at each end, one station for transmitting and one station for receiving the optical signal including a photodetector (PD). In bi-directional optical links, a transmitter and a receiver sub-system is deployed at both sides of the link.
Although FSO communications offers an orders-of-magnitude increase in transmission capacity, while simultaneously reducing antenna size compared to that of modern radio-frequency (RF) technology, atmospheric turbulence generally distorts the wavefront, resulting in both amplitude and phase errors at the detector. Several methods to combat turbulence for FSO have been investigated including arrayed incoherent receivers, pulse-position modulation signaling with coherent arrayed receivers, and digital coherent arrays with electronic wavefront correction.
The state of the art in FSO communications is dominated by the use of adaptive optics (AO) to correct for wavefront distortions caused by atmospheric turbulence, followed by optically single-mode pre-amplified receivers. If the wavefront correction is perfect, the system can restore the ideal receiver sensitivity at 38.3 photons/bit for on-off keying (OOK) modulation. However, AO FSO systems are expensive and have large size, high weight, and high power consumption. More importantly, AO FSO systems still leave much to be desired in terms of reliability since AO does not provide perfect wavefront correction due to the limited throw, limited spatial resolution of the optics, and limited response time making such a system generally inadequate to follow rapid changes in turbulent conditions. As a result, the theoretical sensitivity limit for AO FSO systems is rarely achieved in practice.